Thick film dielectric structures as exemplified by U.S. Pat. No. 5,432,015 (the entirety of which is incorporated herein by reference) are typically fabricated on ceramic or glass substrates and provide superior resistance to dielectric breakdown, as well as a reduced operating voltage compared to thin film electroluminescent (TFEL) displays fabricated on glass substrates. Traditionally three phosphor materials, red, green and blue phosphors, are coated and patterned on top of thick dielectric structure to generate red, green and blue light. However this triple patterning method has several disadvantages such as difficulty in matching turn-on luminance-voltage characteristics, requirement of near perfect uniformity of all three phosphors to match color balance, low yield, long cycle time and high cost of equipment.
Color organic light emitting diode (OLED) displays are known and described, for example in: T. Shimoda et al., Society for Information Display 99 Digest, pp 376-80; U.S. Pat. App. Pub. No. 2002/0043926; C. Hosokawa et al, Society for Information Display 97 Digest pp 1073-6; and U.S. Pat. No. 6,608,439. In particular, U.S. Pat. No. 6,608,439 describes an OLED incorporating semiconductor nanocrystal layers to produce different colors. However, OLEDs cannot be used to build a passive matrix large area display having several pixel rows with any reasonable luminance. This limitation may be mitigated to some extent by using active matrix addressing, but the thin film transistor (TFT) array needed for active matrix addressing is, in itself, difficult to scale up and costly for large area displays with a large number of addressable rows.
U.S. Pat. No. 5,670,839 describes an electroluminescent device that utilizes photoluminescent materials to convert ultraviolet light to visible light. The conversion efficiency for such materials, using ultraviolet excitation, is relatively low. In addition, ultraviolet light tends to degrade the display.
U.S. Pat. No. 5,605,761 describes a homogeneous film comprising a polycarbonate polymeric matrix, dye selected from thioxanthone, perylene imide and thioindigoid compounds and hindered amine light stabilizer (HAL). The film is used to form a fluorescent article which is not degraded by UV light from the ambient environment. U.S. Pat. App. Pub. No. 2003/0015689 describes fine particles of a homogenous material comprising fluorescent coloring matter, a UV absorbent (UVA) and a light stabilizer in a polymer binder. U.S. Pat. App. Pub. No. 2003/0104235 describes stabilizing fluorescent compositions with HALS and UVAs. U.S. Pat. App. Pub. No. 2003/0111641 describes pigments of a homogenous solid material dissolved in a polyamide matrix material containing UVAs and HALS. The compositions comprise dyes dissolved in polyvinyl chloride. None of these aforementioned patents or patent application discloses a stabilized pigment particle composition provided within a photo-patternable resin.
The Applicant's International Patent Application PCT CA03/01567 discloses an electroluminescent display incorporating a blue light-emitting electroluminescent sub-pixel array used in conjunction with color-converting photoluminescent films. In the display, emitted blue light for blue sub-pixels is converted to red light for red sub-pixels and to green light for green sub-pixels. This construction of display overcomes the control difficulties and manufacturing complexities associated with displays constructed according to the prior art. However, it is still necessary to pattern the red and green light-producing photoluminescent films to form the red and green sub-pixel arrays. Further, any differential changes in the efficiency by which these photoluminescent films convert the blue light to red and green light can potentially negatively affect the color balance of the display by causing the luminance of the red and green sub-pixels to decrease at different rates with respect to each other and with respect to the luminance of the blue sub-pixels. The photoluminescent layers are formed by dispersing color-converting fluorescent pigment powder in a UV curable resin which is then screen printed on top of a passivation layer and UV cured. Fluorescent pigments are different from conventional pigments which are organic crystals and which do not fluoresce. The fluorescent pigments absorb high energy photons such as ultraviolet light or short wavelength visible light and emit visible light of lower energy or longer wavelength. Commercially available fluorescent pigments are known to be unstable when they are exposed to sunlight and therefore their light emitting properties, or light-fastness is poor. Where a blue light source is used for excitation of the pigment, it has been found that the available pigments were rendered unstable in the presence of blue light, even in the absence of ultraviolet light. This is partly due to the fact that the absorption of UV light by the fluorescent pigments as provided within polymers can lead to photochemical reactions producing free radicals followed by photo-oxidation.
It is therefore highly desirable to provide an improved color converting photoluminescent film that can be used in an electroluminescent display in a cost effective and operationally effective manner that obviates the shortcomings of the prior art.